Optical lithography is a semiconductor processing technique that uses light to transfer a pattern onto a silicon wafer. However, optical lithography cannot meet reduced pattern size requirements as devices become smaller. Immersion lithography is currently being used to transfer patterns that are too small for standard optical lithography methods. In immersion lithography, a pattern is projected onto a semiconductor substrate coated with a photoresist layer, while a space in between a lens and the photoresist layer is filled with an immersion liquid (e.g., ultra pure water).
In a typical configuration, the immersion liquid is supplied on one side of an optical immersion head assembly and drained at the other side of the optical immersion head assembly as shown in FIG. 1 (presented by B. J. Lin on Arch Interface 2003). Typically, the semiconductor substrate can move relative to the optical immersion head assembly, while the immersion liquid layer is maintained and kept confined to a zone under treatment.
One of the critical aspects of immersion lithography is the formation of gas bubbles at an interface between the photoresist and the substrate. The gas bubbles have been found to be detrimental as they locally distort the optical beam. In order to suppress bubble formation, the (dissolved) gas content of the liquid needs to be kept low.
Another approach to minimizing bubble formation is to reduce the contact angle of the fluid on top of the photoresist layer, or in other words, to improve the wetting of the photoresist by the immersion liquid. This can be done by using photoresist layers with a more liquid-philic (i.e., a liquid with a small contact angle) top surface. However, this approach makes the confinement of the liquid to the zone under treatment much more difficult. As a result, an undesired leakage from the zone between the optical immersion head assembly and the substrate may occur. Such liquid left behind on the treated area then typically evaporates, which is also undesirable. In the field of wafer cleaning, it is well established that improper wafer drying (involving excess evaporation) can typically lead to characteristic residues left behind, often referred to as drying marks or water marks.
The current invention provides extra latitude in the trade-off between gas-bubble formation because of poor wetting of the substrate by the immersion liquid on the one hand and the occurrence of immersion liquid leakage and liquid evaporation on the other hand. Furthermore, the current invention reduces the risk of drying residue formation.